For liquid crystal display elements, the properties of nematic or nematic-cholesteric liquid crystalline materials are utilized to effect a significant change in their optical properties, such as light transmission, light scattering, double refraction, reflectance or color, under the influence of electrical fields. The function of such display elements is based, for example, on the phenomenon of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the nematic-cholesteric phase transition.
For industrial application of these effects in liquid crystal display elements, liquid crystalline materials are needed which have to meet a large number of requirements. Particularly important requirements are chemical stability towards moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet ranges and steady and alternating electrical fields. Furthermore, a liquid crystal mesophase in the temperature range from at least 0.degree. C. to +50.degree. C., and preferably from -10.degree. C. to +70.degree. C., and a viscosity at room temperature of not more than 60 cP are demanded for liquid crystalline materials which can be used industrially. Finally, these materials must not have any characteristic absorption in the range of visible light, that is to say they must be colorless.
A number of liquid crystalline compounds are already known which meet the stability requirements demanded of dielectrics for electronic components and which are also colorless. These include, in particular, the p,p'-disubstituted phenyl benzoates described in German Offenlegungsschrift No. 2,139,628, the p,p-disubstituted biphenyl derivatives described in German Offenlegungsschrift No. 2,356,085 or the phenylcyclohexane derivatives described in German Offenlegungsschrift No. 2,636,684.
These categories of liquid crystalline substances, and also other categories of liquid crystalline substances used hitherto in practice, are built up from aromatic or hydroaromatic rings which have terminal alkyl and/or alkoxy groups and are linked to one another directly or via a bridge group. This structure ensures the rigidity of the molecule necessary for a liquid crystalline mesophase, and in particular a nematic mesophase, to occur. However, many of these compounds have relatively little polar character and are therefore frequently not suitable for dissolving polar doping agents and/or dichroitic dyes in the concentrations which are desired for some types of applications.